This invention relates to an apparatus and method for joining fabric, particularly for joining separate pieces of layered fabric, by means of a waterproof seam in a single operation.
The apparatus and method of the invention allows separate pieces of fabric (woven, non-woven or laminated) to be joined with a strong, flexible, waterproof seam in a single operation.
Presently, many fabrics are sewn together with thread. If waterproofing of the seams is required, this is achieved by applying a sealing tape. This is a non-porous adhesive tape which covers the seam completely, and prevents moisture from passing through the gap between the fabric pieces, through the needle holes or along the threads. Making such a seam is a two-stage process, with the seam first being sewn, and then later being waterproofed with sealing tape. Such a process is time consuming and costly.
An alternative method of joining thin sheets of thermoplastic is by welding the sheets together. Such welds are formed by overlapping the sheets of material and then fusing them by heating. This is done by using heated rollers or knives, or by ultrasonic action, as is disclosed by Canadian Patent Application No. 2,056,812.
Adhesives may be used to join overlapping sheets of material to form a seam, but the practice is uncommon, due to problems in reliability, surface preparation, and achieving uniform adhesive spread.
Overlapping welded or glued bonds are problematic for laminated fabrics. When the seam is flexed, any loads are transferred through the adjacent outer laminations of each piece of fabric (see FIG. 1). This results in an uneven stress distribution over the cross-section of the fabric, which leads eventually to de-lamination. IN the case of laminated fabrics, better stress transfer would be obtained by joining the fabric pieces end-to-end, with adhesive, so that loads could be transferred evenly across the bond in contrast to the prior art. For example, Canadian Patent Application No 2,056,812 discloses a method of joining sheets of a non-woven fabric in an overlapping or face to face configuration (see FIGS. 2 and 3 therein and page 2, lines 12-17 therein) applying a hot-melt adhesive through a glue nozzle, passing the sheets over a patterned anvil roll and heating the joint by means of an ultrasonic horn which transmits mechanical vibrations to the sheets to be joined. This method does not function well with fabrics other than very light, thin non-woven fabrics.
It is an object of the present invention to provide an apparatus and method for joining fabric in a single operation, which is suitable for use with laminated fabrics, which promotes penetration of the fabric by the adhesive, and which allows a strong bond to be made.
According to a first aspect of the invention there is provided a seam bonding apparatus comprising a hot-melt adhesive supply means, an ultrasonic energy source and a sonotrode adapted to direct ultrasonic energy to an application point on the seam to be bonded, wherein the sonotrode has therein a passage adapted to permit flow therethrough of adhesive from said hot-melt adhesive supply means to said application point.
Preferably the apparatus further comprises a nozzle communicating with said passage. Preferably said nozzle is adapted to be less than 5 mm from the application point, most preferably less than 2 mm from the application point.
According to a particular embodiment said nozzle is integral with said sonotrode. Preferably said passage comprises a first axial portion. Preferably said passage further comprises a substantially radial portion communicating with said axial portion.
According to a further particular embodiment the apparatus comprises a glue chamber adjacent to said sonotrode. Preferably said passage and said nozzle each communicate with said glue chamber. Preferably said glue chamber has a parabolically shaped inner surface.
Preferably said hot-melt adhesive supply means comprises a hot-melt adhesive tank and a pump. Preferably the apparatus further comprises a heated delivery pump connecting said pump to the passage.
According to a second aspect of the invention there is provided a method for producing a bond between members of an article, said method comprising the steps of:
heating a hot-melt adhesive until it is molten,
feeding said molten adhesive to an application point in which said adhesive contacts said members,
applying ultrasound energy by means of a sonotrode to said molten adhesive, and
allowing said molten adhesive to cool and thereby form a bond between said members.
Preferably mechanical vibrations are induced by the ultrasound energy in said members to aid the formation of the bond.
Preferably the molten adhesive is fed through a passage in said sonotrode.
Preferably the molten adhesive is fed onto the application point through a nozzle positioned such that there is a small gap between said members and said nozzle. Preferably said gap is less than 5 mm, most preferably less than 2 mm.
Preferably said members are positioned in an abutting position, such that a butt bond is produced between said members.
Preferably the method uses an apparatus according to the first aspect of the invention.
According to a third aspect of the invention there is provided an article comprising a plurality of members and having a bond produced between two of said members by the method according to the second aspect of the invention.
The method and apparatus of the invention offer significant advantages over the prior art. The method poses little threat to the operator, and the following effects of using ultrasound aid the formation of a good bond:
a. Energy can be transmitted via the molten adhesive to the seam. This energy can also be focused.
b. Ultrasound causes heating and softening of solid polymer, such as the fabric fibres in contact with the adhesive. Crystallisation of the molten adhesive is also inhibited.
c. Ultrasound can cause cavitation in liquids. There are two manifestations:
i. Cyclical expansion and contraction of stationary voids. This causes pumping of the surrounding liquid, to form microstreams.
ii. Random formation of large voids which subsequently implode, causing high pressure transients.
These effects encourage separation of the fabric fibres and active infiltration by the adhesive.
The fabric may be preheated to achieve a good bond with the holt-melt adhesive.
The process is particularly effective when used with composite fabrics having between two and four layers with a total thickness of 200-500 xcexcm. With such fabrics the method can achieve seam sealing rates in the range 2-8 m/min (0.033-0.33 m/s).
Seam sealing according to the invention can be carried out with composite fabrics whose outer layers are commonly polyester, nylon, acrylic or polyester-cotton. The composite fabrics may include semi-permeable membranes of polyamide or polyurethane.
It is possible to extrude the adhesive into a thread, allowing it to be fed to the seam as a solid. Different diameters of thread may be matched with different thicknesses of material, to be transported by the same feed mechanism as the fabric.
Cooling may be by forced convection, using the cold air-stream from a vortex tube. The hot air by-product of the tube may be used for preheating the fabric.
Peltier heat pump assemblies are available, rated at tens of Watts. These may be attached to compression plates, recessed to avoid contact with the adhesive, which remove heat from the fabric by conduction, and from the melt by radiation and conduction via the fabric.